1. Field of the Invention
The present invention is directed to a catalyst curing system and method of using the same to reduce emissions or xe2x80x9cfumingxe2x80x9d of formaldehyde in resorcinol-formaldehyde or phenol-resorcinol-formaldehyde resins. The present invention replaces a portion of oxazolidine, normally used for crosslinking of a resorcinol or phenol-resorcinol resin with a class of compounds normally called methylolureas, which are reaction products of formaldehyde and urea chemicals.
2. Description of the Related Art
Resorcinol-formaldehyde and phenol-resorcinol-formaldehyde resins are used as adhesives, such as in the wood products industry, to manufacture a variety of exterior rated laminated wood items. They are noted for their excellent exterior bonding properties, and their ability to fully crosslink and cure at temperatures as low as 60xc2x0 F. The latter is important since most items produced with these adhesives in the wood products industry do not lend themselves to heated cure because of size or geometry. Both of these resins are manufactured with a deficiency of formaldehyde, to prevent premature and uncontrolled cure or gelation of the resin. In use they are catalyzed by adding a sufficient amount of additional reactant, normally formaldehyde in polymer form called paraformaldehyde, which causes the adhesive to thicken and gel, and eventually cure completely at room temperature. A major drawback to this method of catalysis is the off-gassing, or xe2x80x9cfumingxe2x80x9d, of formaldehyde vapors from the adhesive while it is being used to manufacture a wood article. Plant workers may be exposed to these vapors, and this may pose a significant health risk under current OSHA regulations. One means of alleviating the problem of formaldehyde exposure in the workplace is the use of a methylene donor, or crosslinking agent, that does not release formaldehyde gas. One such material is a class of compounds known as oxazolidines. These compounds are the reaction product of formaldehyde and certain amino alcohols. When mixed with resorcinol or phenol-resorcinol resin they release formaldehyde to the resin for curing, but do not release a discernible amount of formaldehyde gas to the atmosphere. This makes them preferable to formaldehyde solution or paraformaldehyde as crosslinkers for the adhesives. The oxazolidines have two major drawbacks that prevent widespread acceptance in the marketplace, however. First, the gel times achieved with oxazolidines are generally much shorter than those obtained with conventional paraformaldehyde catalyst. This limits the plants that can use the oxazolidine catalyst to those with fast article assembly, such as wood I-beam processes. Secondly, the oxazolidine is much more expensive than formaldehyde solution or paraformaldehyde. Some plants that would like to have the low xe2x80x9cfumingxe2x80x9d properties of the oxazolidine catalyzed glue are unwilling to pay the additional price for this improvement.
For these reasons there is a long felt need for an improved low xe2x80x9cfumingxe2x80x9d catalyst system for resorcinol and phenol-resorcinol resins. The improvement should address problems inherent in the oxazolidine catalyst, namely, too rapid gelation and cure, and high cost. The improvement should lead to better control of cure time and lower cost, while maintaining the low xe2x80x9cfumingxe2x80x9d attribute of oxazolidine catalyst.
It is an object of the invention to provide an improved low xe2x80x9cfumingxe2x80x9d catalyst system for resorcinol and phenol-resorcinol resin systems.
It is a further object of the invention to provide methods of curing resorcinol and phenol-resorcinol resin systems where too rapid gelation and cure, and high cost, are avoided.
It is a still further object of the invention to provide improved adhesives and wood products employing them which have superior properties, e.g., bonding performance.
It is another object of the invention to provide adhesive compositions which have low fuming characteristics.
These and other objects will be more fully understood in conjunction with the following description of the invention.
We have discovered that replacement of a portion of the oxazolidine normally used for crosslinking of a resorcinol or phenol-resorcinol resin leads to improved performance of the adhesive, while substantially reducing the cost of the adhesive. The replacement material for the oxazolidine is the class of compounds normally called methylolureas, i.e., reaction products of formaldehyde and urea chemicals. These compounds do not xe2x80x9cfumexe2x80x9d formaldehyde, and also function as methylene donors for resorcinol or phenol-resorcinol resin, though the reactivity is such that room temperature cure of the resin cannot be achieved with conventional resin technology. There appears to be a synergy developed with the mixture of oxazolidine and methylolurea wherein the gluing performance is enhanced, and cost is substantially reduced.
There are two major types of wood products that use room temperature curing resorcinol or phenol-resorcinol adhesives. These are flat laminated beams, where lumber of various dimensions is laminated into structural load-bearing members, and wood I-beams, where lumber, plywood, laminated veneer lumber (LVL), and oriented strandboard (OSB) are glued into composite structural members. The assembly of flat laminated beams requires the use of adhesives that have a working life, or pot-life, of 120-240 minutes. This working life is achieved with paraformaldehyde catalysts by using a moderated paraformaldehyde with reduced reactivity. Wood I-beams however, are assembled by machine, and the adhesive is usually mechanically mixed immediately prior to application. This operation actually benefits from the use of a short working life and fast gel time adhesive. An oxazolidine catalyst works very well in the I-beam application, aside from the high cost of the adhesive. When an amount of oxazolidine sufficient to cause complete crosslink cure of the resin is used, the working life of the adhesive mix is usually 60 minutes or less. This is too short for use in the flat beam laminating process, where the beam is often assembled and clamped by hand. It is not possible to extend the working life of the oxazolidine catalyzed adhesive by merely reducing the level of catalyst, since an amount sufficient to cause complete crosslink cure of the resin is necessary. Other methylene donors such as nitro alcohols and hexamethylenetetramine (hexa) have been used in the past as crosslinkers for resorcinol or phenol-resorcinol resin. They are not used to any great degree in wood bonding because of poor bonding performance or high cost. In the case of hexa, ammonia is generated during the curing process, and is emitted from the glue.
By using a catalyst composed of a mixture where part of the oxazolidine is replaced by methylolurea, several improvements are realized. Working life of the adhesive mix is proportional to the amount of methylolurea substituted for oxazolidine. This greatly improves the bonding performance of the adhesive when doing flat beam laminating, as working life up to 180 minutes is easily achieved. Methylolurea does not emit a volatile gas when it functions as a methylene donor to the resin, thus noxious emissions are minimized. Methylolurea is inexpensive, on the order of $0.10/lb., compared to $1.50/lb. or higher for oxazolidine.
Laboratory tests have demonstrated the superior bonding performance of the oxazolidine/methylolurea mixture when compared to oxazolidine alone.